The present invention relates to a brushless DC motor with armature windings compensated by auxiliary windings, and particularly to a brushless DC motor with armature windings compensated by auxiliary windings wound around the linking parts of each pair of adjacent teeth of the slotted stator to increase the number of effective turns, thus improving the torque constant and efficiency of the motor.
Brushless DC motors are different from regular DC motors in having a rotating permanent magnet type. They have been widely used as typical motors for multimedia equipment used with computers, such as peripheral equipment (HDD, CD-ROM, DVD), video cassette recorders (VCR), camcoders. Also of late, they have been used for driving ultracompact portable information storage devices such as IBM microdrives developed by IBM Corp. In the case of using small precisional motors in high value-added products such as computer hard disk drives, fluid bearings are increasingly used instead of ball bearings as the former is excellent in mechanical characteristics which reduce noise and vibration. However, the use of fluid bearings requires bigger torque than that of ball bearings during starting period, due to the viscosity and consequent friction of the fluid.
FIG. 1a is a plan view showing a brushless DC motor of the inner rotor type with twelve poles and nine slots, and FIG. 1b is a plan view showing a brushless DC motor of the outer rotor type with twelve poles and nine slots, according to a prior art. As shown in FIG. 1a and FIG. 1b, the motor is classified into the inner rotor type and outer rotor type based on the relative position of the rotor 40 with permanent magnets and the stator 10 with slots between each pair of adjacent teeth.
The brushless DC motor of the inner rotor type comprises a stator 10, a rotor 40 inside the stator, and air gaps 30 between the stator and the rotor. The stator comprises nine contiguously formed teeth 11, main windings 20 wound around the nine teeth 11 of the stator 10, and the rotor comprises twelve poles of permanent magnets 41 and a yoke 42.
The brushless DC motor of the outer rotor type comprises a stator 10, a rotor 40 outside the stator, and air gaps 30 between the stator and the rotor. The stator comprises nine contiguously formed teeth 11, main windings 20 wound around the nine teeth 11 of the stator 10, and the rotor comprises twelve poles of permanent magnets 41 and a yoke 42.
FIG. 2a is a three-phase, Y-connection diagram showing the windings wound around the teeth of the stator having nine slots of a brushless DC motor of the inner rotor type or outer rotor type, according to a prior art.
The concentric windings are made in series in the following orders: for A phase 50, A1 tooth 11a, A2 tooth 11b, and A3 tooth 11c, for B phase 60, B1 tooth 11d, B2 tooth 11e, and B3 tooth 11f, and for C phase 70, C1 tooth 11g, C2 tooth 11h, and C3 tooth 11i. 
As shown in FIG. 2a, each connected winding from the A3 tooth 11c, B3 tooth 11f and C3 tooth 11i is connected together at the neutral point 80 to form a Y-connection.
FIG. 2b is a three-phase, -connection diagram showing the windings wound around the teeth of the stator having nine slots of a brushless DC motor of inner rotor type or outer rotor type, according to a prior art.
The concentric windings are made in series in the following orders: for A phase 50, A1 tooth 11a, A2 tooth 11b, and A3 tooth 11c, for B phase 60, B1 tooth 11d, B2 tooth 11e, and B3 tooth 11f, and for C phase 70, C1 tooth 11g, C2 tooth 11h, and C3 tooth 11i. 
As shown in FIG. 2b, each connected winding of A phase, B phase and C phase is connected together to form a -connection.
FIG. 2c is a plan view of a brushless DC motor and magnetic equivalent circuit diagram showing the direction of the magnetic flux flowing through the closed path of flux in an inner-rotor-type brushless DC motor with twelve poles, nine slots and Y-connection, according to a prior art.
As shown in FIG. 2c, the concentric windings of the slotted stator are made in series in the following orders: for A phase, tooth A1, tooth A2, and tooth A3, for B phase, tooth B1, tooth B2, and tooth B3, and for C phase, tooth C1, tooth C2, and tooth C3. In a state of three-phased connections of A phase, B phase and C phase, when currents flow into two of the phases, a flux flows through the closed path. The flux passing through the air gap is determined by the following equation 1.
"psgr"m={NmIm/(Rair+Rmag)}+{"psgr"rRmag/(Rair+Rmag)}xe2x80x83xe2x80x83(Equation 1) 
Nm: number of turns of the main windings
Im: current of the main windings
Rair: magnetic reluctance of the air gap
Rmag: magnetic reluctance of the permanent magnets
"psgr"r: flux output by permanent magnets
NI: magnetomotive force
In the above equation, the magnetomotive force induced by main windings 20 is NmIm, and the magnetomotive force made by permanent magnets is "psgr"rRmag.
In the brushless DC motor constructed as described above, the position of the rotor is determined by a sensor such as a hall sensor or an encoder, or a sensorless method of using variations of the back electromotive force or inductance, so that the direction of the current flowing into each phase of being commutated is determined, and a rotating magnetic field is created.
Brushless DC motors of the inner rotor type are easy to control and radiate heat effectively, but have considerable variations of speed because of their small rotating inertia, while brushless DC motors of the outer rotor type have little variations of speed because of their large rotating inertia, but have relatively poor heat radiation.
Recently, as brushless DC motors using permanent magnets made of a neodymium-iron-boron of high coercive force become smaller and thinner, the thickness and height of the permanent magnets have become smaller, and the length and width of the teeth of the stator have become shorter and narrower, resulting in restricting the room for the windings accordingly. This problem has resulted in decreasing the torque constant of the brushless DC motor which is be determined by the windings of the stator and the permanent magnets of the rotor. A decrease in the torque constant brings about a decrease not only in the starting and the driving torque of the brushless DC motor, but also in the efficiency of the brushless DC motor. An example is the ultracompact, low speed spindle motor used in a microdrive, in the range of the operating speed (in case of IBM microdrive, 3600 or 4500 rpm). In particular, in a system which requires low power such as a portable information storage device, low efficiency is an essential problem which shortens the lifetime of the system. Accordingly, there is a need for an ultracompact brushless DC motor having improved starting/driving torque and efficiency, with increasing torque constant.
The present invention is made to solve above problems. The purpose of the present invention is to provide a brushless DC motor compensated by auxiliary windings wound around the linking parts of each pair of adjacent teeth of the slotted stator to increase the number of effective turns for each phase, thus improving the torque constant and efficiency of the motor.
A characteristic of a brushless DC motor with armature windings compensated by auxiliary windings according to the present invention for attaining the above mentioned technical object is that:
the motor comprises a slotted stator formed with a number of teeth, main windings concentrically wound around the teeth of said stator, auxiliary windings for compensating the main windings wound concentrically around the linking parts of said teeth and connected to said main windings.